The present invention relates to a linear motion device and to a method for inserting and withdrawing control rods, and more particularly to a control rod drive mechanism (CRDM) capable of independent and sequential positioning of two sets of control rods with a single motor stator and rotor.
Control rod drivers of the general class to which the present invention is related are disclosed in:
U.S. Pat. No. 3,946,258 to Leshem PA1 U.S. Pat. No. 3,882,333 to DeWeese PA1 U.S. Pat. No. 3,825,160 to Lichtenberger et al. PA1 U.S. Pat. No. 3,822,439 to Wallin et al. PA1 U.S. Pat. No. 3,599,498 to Misenti et al. PA1 U.S. Pat. No. 3,583,254 to Winders et al. PA1 U.S. Pat. No. 2,780,740 to Roman et al.
Control rods are generally used in nuclear reactors for controlling the output thereof. These rods must be finely adjusted for changes in output and for maintaining the output at desired levels. In the event that the reactor must be rapidly shut down, these control rods must also be capable of scramming, which requires that they be fully inserted into the reactor core as rapidly as possible.
These control rods must be driven by power from some source outside the reactor. Since the control rods are within the reactor core and, therefore, within the pressure vessel, the driving force must in some way be transmitted through the walls of the pressure vessel by using seals or some drive which permits a hermetic seal.
A large number of control rod drives are used within a single-reactor core. These are relatively close to one another, creating space problems, particularly where large non-cylindrical forms are used.
A known type of linear drive, particularly useful for control rods, operates on the principle of a spinning nut which is rotatable but held against axial movement and which meshes with a screwshaft that is moved longitudinally when the nut is rotated. Such a device is described, for instance, in U.S. Pat. No. 2,780,740 to Roman et al. or U.S. Pat. No. 3,822,439 to Wallin. In the known construction, the nut is formed of a plurality of roller-type nut sections, each carried by one arm of a lever of the first order and movable radially with rocking movement of the levers. The other arms of the levers constitute segments of the rotor of a synchronous reluctance motor. The levers are rockable in radial planes under the influence of the magnetic field of the motor and such rocking movement is effective to carry the roller sections into and out of mesh with screwshafts. The levers are biased in a direction which disengages the rollers but are held in the roller-meshing position by the magnetic field of the motor.
Under normal conditions, the field coils of the motor are energized at all times. When the load, such as a control rod, is to be moved, the field is rotated in the desired direction. When in the hold mode, the field is energized but stationary and the screwshaft and control rod are held against movement by the stationary nut sections, the threads of the nut sections and shaft being at a wedging angle so that the nut cannot be rotated by gravity. For scram, or in an emergency such as might occur in event of a power failure, the field coils are deenergized, the magnetic field collapses, and the screwshaft and rod are free to drop into the reactor.
Commercial nuclear reactors in general and light water breeder reactor (LWBR) designs in particular may employ a large number of identical fuel assemblies, each of which may be controlled by a separate single action control rod drive mechanism. Each fuel assembly of an LWBR may contain three types of control rods: (1) fixed, unmoving rods, (2) movable poison shutdown rods, and (3) movable thoria control rods. This general concept has been termed thoria finger control.
With regard to the thoria finger control concept and also for other control rod arrangements, for purposes of mechanical and hydraulic design and for physics and nuclear considerations, it would be advantageous to have a control rod drive mechanism capable of independently positioning two sets of rods on concentric lead screws. To date, this requirement can only be satisfied by piggybacking, upon each other, two "standard" single acting mechanisms of the general type discussed above. However, the resulting overall height becomes excessive. Additional problems from such an arrangement result from the fact that the loads carried by the uppermost mechanism must be transmitted though the motor tube of the lower mechanism, leading to additional mechanical design problems.
All of the patent references referred to above are designed to control only a single control rod lead screw. The inventions disclosed in U.S. Pat. Nos. 2,780,740; 3,585,254; 3,599,498; and 3,822,439 all relate to the standard collapsible rotor control drive mechanisms. Each controls only a single lead screw with either a scram or no scram on loss of power feature built in. There are, of course, situations where either of these features are desired. These collapsible rotor design control rod drive mechanisms have a rotating cage enclosing segment arms which carry roller nuts to engage a lead screw in a manner similar to the present invention. However, none of these mechanisms are applicable where there is a need to axially position two concentric control rod leadscrews, unless two cage assemblies, motor coils, etc. are positioned one atop the other, which is a very undesirable situation, both from a size and mechanical design standpoint.
In fact, such a piggyback arrangement would more than double the height of the control rod drive mechanism and the complexity of the control drive power supplies and cooling systems. For a reactor core height of twelve feet, the resulting mechanism length would be 27-30 feet.
The inventions disclosed in U.S. Pat. Nos. 3,825,160; 3,882,333; and 3,946,258 all relate to the design or improvements of a type of jack with a plurality of latches alternately engaging and disengaging a long "jackshaft" which takes the place of the leadscrew in the collapsible rotor design. Although this design is in very common use in commercial reactors, it is not useful if two concentric shafts or leadscrews need to be positioned unless, again, the height of the mechanism is more than doubled. Moreover, this design is inherently incapable of exerting any inward force on the control rods, which is desirable in many applications.